Does the commit history of your source repository look like this:
f02af91822 docs: fix incorrect subheadings dd7bee8b38 cli: add --import option 900ca2936a cli: extract move_topic() helper function
Or like this:
7011cc9868 lunch time a07c82331d resolve code review comments 331d79a8ff more fixes
?
The first is a clean git commit history where each commit has a clear purpose and is a single logical change. The second is a messy commit history where the commits have no inherent structure:
- "more fixes" does not describe clearly what is being fixed and the plural ("fixes") hints it may contain multiple logical changes instead of just one.
- "resolve code review comments" contains changes requested by code reviewers in relation to another commit, it's not a self-contained logical change.
- "lunch time" is an unfinished commit that was created because the programmer wanted to save their work.
Commit anti-patterns
The example above illustrates several anti-patterns:
Vague commit messages
If the commit message is vague and does not express a clear purpose, then it is hard to know what a commit does from the commit message. If git-log(1) doesn't provide useful information about commits then one has to resort to searching the code diffs. That is very tedious and sometimes it's almost impossible to come up with a good code search query while a clear commit message would have been easy to search. So clear commit messages are the first step towards clean commit history.
Doing too many things in one commit
Commits that make several logical code changes are hard to review and impede backporting fixes to stable branches. For example, a commit that fixes a bug as well as adding a new feature may need to be rewritten for a stable branch. If instead the code had been split into two commits, then the bug fix commit could have been backported easily. Therefore it is good practice to separate distinct bug fixes, features, and other logical code changes into separate commits.
Addressing code review comments
The code review and testing history is usually not useful information once a commit has been merged. For example, if there was a continuous integration (CI) test failure and a pull request needed to be changed, then the change should be made directly to the buggy commit so that the final commit passes the tests. No one needs to know about the code review or testing history once the code is merged and keeping these artifacts makes the commit history unwieldy by spreading a logical code change across multiple incomplete commits.
Saving work
There are valid reasons to temporarily save your work in a commit, but work-in-progress (WIP) commits should be cleaned up before merging them. For example, sometimes people make arbitrary commits to save work at the end of the day. That is fine in a local branch, but those temporary commits can be restructured into clean commits using git-rebase(1). No one else needs to know about temporary commits.
Broken commits
It can be easy to accidentally include a commit that does not build or fails tests if a later commit happens to resolve the issue. Since the later commit hides the issue it may not be apparent when testing the branch. When reordering commits the risk of introducing broken commits increases because those commits were originally written in a different order. I use the git-rebase(1) exec action to build and run tests after every commit to detect broken commits when doing extensive rebases.
Why clean commit history is important
Not all reasons for maintaining a clean commit history are obvious. Unfortunately all the above anti-patterns make commit history less useful so it's interesting to note that if you value any of the following reasons for keeping a clean commit history, then all anti-patterns need to be avoided.
Code review
Reviewers have an easier time reading clean commits than an unstructured series of commits. For example, if there is a broken commit because a function is used before it is defined in a later commit, then that affects code reviewers who read the commits linearly. They will be puzzled by the non-existent function and unable to decide whether it is being used correctly because it has not been defined yet. Although code reviewers could put in extra effort to reread the commits multiple times and try to remember the misordered changes, it's better to let code reviewers spend time on real issues rather than on untangling poorly structured commits.
Capturing the rationale for code changes
When each commit is a single logical change it becomes possible to write good commit descriptions that give the rationale for the code change. Explanations for why a code change is necessary, as well as links to issue trackers, email discussions, etc can be valuable when revisiting the commit history later. If commits contain multiple logical code changes or are incomplete then it is hard to include a good commit description, so the commit history is less useful when referring back to it later on.
Making cherry-picking easy
Many software projects maintain stable branches that still receive bug fixes for some time. This allows development to introduce new features and less mature code while users can run a mature stable release. However, maintaining stable branches can be time-consuming. Maintainers need to identify commits suitable for stable branches and cherry-pick or backport them. This requires clean commit history so that bug fixes can be applied in isolation without dragging in other code changes that do not fit the criteria for stable branches.
Enabling git-bisect(1)
When a bug is observed it may not be clear which commit introduced it. The git-bisect(1) command systematically searches the commit history and identifies the commit that caused the bug. However, git-bisect(1) only works with clean commit history. If there are broken commits then bisection becomes unreliable because some portions of commit history cannot be tested. Poorly structured commits, such as huge changes that do many different things, also make it difficult to identify which line caused the bug even when git-bisect(1) has determined which commit is to blame.
When clean commit history does not matter
The reason I have found that not everyone practices clean commit history is that they may not need any of this. Especially small projects developed by a single author may involve little code review, backporting changes to stable branches, or git-bisect(1). In that case the effort required to split code changes into clean commits and write good commit messages may seem unjustified. Of course this can change but once the commit history is messy there is not much to be done. So it's worth thinking carefully about whether to take shortcuts.
Another factor is poor tooling. Gerrit and GitHub's code review has historically made it hard to practice clean commit history. They were not designed for reviewing commit series and favored anti-patterns like squashing everything into a single commit or adding additional commits to address code review feedback. These are tool limitations and luckily GitHub code review has become better over the years. Tools that encourage you to review a commit series as a single diff are not conducive to clean commit history.
Finally, clean commit history requires proficiency with git-rebase(1) and that you are comfortable with the idea of rewriting your local branch to clean it up before publishing it. It takes a little practice to become competent at reordering, squashing, and splitting commits. The process can be a little scary, although git-reflog(1) makes it possible to undo even the most serious errors where commits were accidentally lost. On a related note, some people falsely believe that a pull or merge request branch should not be rebased. Although it is good practice to avoid rewriting history of branches that other people track, rewriting history and force-pushing a pull request is different. Most of the time no one else will maintain a local branch based on it and therefore force-pushing will not inconvenience anyone. Even if it is necessary to develop branches based on someone else's not-yet-merged branches, one needs to weigh the trade-offs of having to do more work in the short-term with the drawbacks of having a messy commit history forever.
Conclusion
I hope this is a useful summary of why each commit should have a clear purpose and embody a single logical change. For source repositories that are used by more than one person it is especially important to think about commit best practices. Clean commit history facilitates better code review, bug-finding, and maintaining stable branches. Beyond that it also provides a useful form of communication and sharing knowledge about the codebase that is missing when commit history is disregarded.